Fail-safe comparator

ABSTRACT

This disclosure relates to a fail-safe comparator circuit having an input transformer, the primary of which is fed by an a.c. signal voltage. The secondary winding of the first transformer is serially connected to the primary winding of a saturable type of output transformer. A current-limiting resistor forms a series circuit with the secondary winding of the input transformer and the primary winding of the output transformer. A separate d.c. input voltage is applied to each end of the series circuit so that a.c. output signals are induced into the secondary winding of the output transformer when and only when the d.c. input voltages are in agreement and no critical circuit or component failure is present.

Umted States Patent [15] 3, Darrow 51 Nov. 14, 1972 [54] FAIL-SAFECOMPARATOR Primary Examiner-Robert K. Schaefer [72] inventor: John O. G.Darrow, Murrysville, Pa. Assistant Emminerwmiam Smith Attorney-H. A.Williamson, A. G. Williamson, Jr. [73] Assignee: Westinghome Air BrakeCompany, and u Swissvale, Pa.

22 Filed: March 4, 1971 [571 ABSTRACT [211 App] 120,993 This disclosurerelates to a fail-safe comparator circuit having an input transformer,the primary of which is fed by an ac. signal voltage. The secondarywinding of "307/2, 8 the first transformer is serially connected to theprimas e u l y i di g f a t bl typ f transformer. [58] M Search "307/2;324/98 A current-limiting resistor forms a series circuit with 324/140the secondary winding of the input transformer and the primary windingof the output transformer. A [56] References Cited separate d.c. inputvoltage is applied to each end of UNITED STATES PATENTS the seriescircuit so that a.c. output signals are induced into the secondarywinding of the output trans- 2,689,328 9/1954 Logan ..324/ l 17 R formerwhen and only when the input voltages are 2,892,155 6/1959 Radus et a1...324/1i7 R in agreement and no critical circuit or component 2,888,6455/1959 Hoft et al ..324/98 X f il is present 2,338,423 l/l944 Geyger..324/l17 R 6 Chins, 1 Drawing Figure npa.

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FAIL-SAFE COMPARATOR My invention relates to a fail-safe circuitarrangement and more particularly to voltage comparator for measuringthe amplitudes of two d.c. input signals and for producing an a.c.output signal when and only when the amplitudes of the two d.c. inputsignals are in agree ment and no critical circuit or component failureis present.

Various types of comparator circuits compare the amplitudes of at leasttwo input signals and produce an output signal when the amplitudes ofthe input signals which are substantially equal are old and well knownin the art. Normally, these previous comparator circuits are generallyacceptable for use in ordinary nonvital applications but are whollyunsatisfactory for employment in special vital operations. For example,in signal and communication systems for railway or mass and/or rapidtransit operations, it is mandatory that each portion or circuit of theassemblage operates in a fail-safe manner. Such a stringent operatingrequirement is essential in order to prevent costly damage to theequipment as well as to preclude serious injury and possible death toemployees and passengers. It will be appreciated that a high speed trainor mass transit vehicle is an imminently dangerous object if the controlsystem fails in an unsafe manner. Therefore, in a vehicle speed controlsystem a less restrictive speed command should never be capable of.being simulated by a circuit or component failure. In order to ensuresuch operation, it is necessary to carefully analyze and examine eachand every circuit component for all its shortcomings, as well as tooptimize the design and layout of the circuit. Thus, it will beappreciated that a comparator circuit which is capable of producing anoutput signal when its input signals are out of agreement is notsuitable for use in a vital type of vehicle speed control system forrailway or mass and/or rapid transit operation.

Accordingly, it is an object of my invention to provide a new andimproved comparator circuit which operates in a fail-safe manner.

Another object of my invention is to provide a failsafe voltagecomparator which produces an output signal only when its'input signalsare in agreement.

A further object of my invention is to provide a vial type of comparatorcircuit which is incapable of producing an output signal during acritical circuit or component failure or when its input signals are notin agreement.

Still another object of my invention is to provide 2 fail-safe voltagecomparator circuit for comparing the level of a pair of d.c. voltagesand producing an a.c.

ages are substantially equal.

Still a further object of my invention is to provide a fail-safe voltagecomparing circuit employing a pair of transformers one of which is fedwith an a.c. input and the other of which produces an a.c. output onlywhen the amplitudes of a pair of d.c. inputs are in agreement.

Yet a further object of my invention is to provide a fail-safe voltagecomparator having a first input transformer and a second outputtransformer which produces an output signal only when the amplitudes ofa pair of input signals are substantially equal and no critical circuitor component failure is present. I

Still yet another object of my invention is to provide a fail-safecomparator circuit which is economical in cost, simple in design,reliable in operation, durable in use, and efficient in service. g

In accordance with the present invention, the unique voltage comparatorcircuit compares the amplitudes of a pair of d.c. inputs and onlyproduces an a.c. output when the amplitudes of the pair of d.c. inputsare in agreement. The comparator circuit includes an input transformerand a saturable core output transformer. A source of a.c. input signalsis applied to the primary winding of the input transformer. A seriescircuit is formed by the secondary winding of the input transformer andthe primary winding of the output transformer along with acurrent-limiting resistor. One of the pair of d.c. inputs is connectedto one end of the series circuit while the other d.c. input is connectedto the other end of the series circuit. Normally, when the pair of d.c.inputs are substantially equal and the circuit is intact, only a.c.current flows through the series circuit so that an a.c. output isinduced into the secondary winding of the saturable transformer. Ifeither d.c. input varies to any degree relative to the other, a d.c.current will flow through the series circuit and cause the saturabletransformer to saturate so that a.c. output signals will not appearacross the secondary winding of the saturable transformer. A componentor circuit failure, such as, an open or short condition, will destroythe integrity of the circuit or prevent any transformer action fromoccurring between the various windings. Thus, a.c. output signals areinduced in the secondary winding of the output transformer when and onlywhen the pair of d.c. inputs are in agreement and no cirtical circuit orcomponent failure is present.

Other objects, features and advantages will be more readily appreciatedas the subject invention becomes better understood by reference to thefollowing 1 detailed description when considered in conjunction with theaccompanying drawing wherein:

The single FIGURE is schematic circuit diagram of a fail-safe electroniccomparator circuit embodying the principles of the present invention.

Referring to the single figure of the drawing, there is shown apreferred embodiment of a fail-safe comparator circuit of the presentinvention. The fail-safe comparator circuit includes a pair oftransformers TI and T2. The first transformer T1 is a conventional typewhich includes a primary winding P1 and a secondary winding SI, both ofwhich are wound on an ordinary magnetic core C1. The second transformerT2 is a saturable type which includes a primary winding P2 and asecondary winding S2, both of which are wound on a magnetic core C2having a square hysteresis loop. That is, the core C2 is easilysaturated and causes the windings P2 and S2 to possess a high impedancewhen little or no d.c. current flows through the primary winding P2 andcauses windings P2 and S2 to have a very low or substantially zeroimpedance when current flows through the primary winding P2 andsaturates core C2.

As shown, one end of the secondary winding S1 is connected to one end ofthe primary winding P2 by lead L1. One end of a current-limitingresistor R is con? nected to the other end of primary winding P2 by leadL2. The other end of resistor R is directly connected to input terminal1 by lead L3. lnput terminal 2 is connected to a point of referencepotential, such as, ground. As shown, the other end of the secondarywinding S1 is directly connected to input terminal 3 by lead L4. Inputterminal 4 is also connected to a point of reference potential, such as,ground.

The input terminals 1 and 2 are connected to a suitable source of d.c.input voltage V1. The d.c. voltage source V1 may be derived of a gatedtype of fail-safe amplifier-rectifier-filter circuit which preferablyhas a relatively low impedance value. Similarly, the input terminals 3and 4 are connected to a suitable source of c.c. voltage V2. Like sourceV1, d.c. source V2 may also be derived from a gated type of fail-safeamplifierrectifier-filter circuit having a relatively low impedancevalue.

It will be understood that the voltage sources V1 and V2 have separateand unrelated origins which may represent different functions. Forexample, source V1 may be representative of the actual speed of a movingvehicle while source V2 may be representative of the last speed commandsignal received by the moving vehicle. If the actual speed is identicalto the last received speed command signal, then the amplitudes of thevoltages V1 and V2 should be substantially equal or, in other words, thevoltages V1 and V2 should be in agreement. This agreement betweenvoltages V. and V2 is checked by applying a suitable a.c. signal sourceto the primary winding P1 of the transformer T1. The a.c. signal sourcemay be any conventional supply source which is readily available at themoment. If voltages V1 and V2 are in agreement, secondary winding S1supplies a.c. signals to the primary winding P2 which, in turn, inducesan a.c. voltage into the secondary winding S2 so thatan output signal isproduced, as will be described in detail hereinafter. If it isnecessary, the a.c. output developed across secondary winding S2 may besubsequently amplified and rectified, and thereafter applied to somesuitable utilization device, such as, a vital relay. Hence, when thevital relay is energized, its front contacts will be closed which isindicative that the voltages V1 and V2 are substantially identical, andthus an agreement indication will result.

Turning now to the operation of the present invention, it will beassumedth'at the circuit is intact and that the actual speed of thevehicle corresponds to the last received speed command signal. Underthis condition, both of the amplifier-rectifier-filter circuits aregated ON and the d.c. voltages, namely, V1 and V2, appearing on inputterminals 1, 2 and 3,4, respectively, are substantially equal. Underthis condition little, if any, d.c. current will flow through the seriescircuit formed by the resistor R and the windings P2 and S1. That is,when voltages V1 and V2 are in agreement, a bucking or opposing actiontakes place so that the'sum of the d.c. current in the series circuit iseffectively zero. Thus, the transformer T2 remains in an unsaturatedstate, and the windings P2 and S2 will have a relatively high impedance.Under this condition substantially all of the a.c. signal voltage willdrop across primary winding P2 so a.c. signals are induced intosecondary winding S2. The a.c. signals appearing across secondarywinding S2 are, in turn, amplified and rectified and are then employedto energize the vital relay. Thus, the energization of the vital relaycauses its front contact to close which may be employed to provide anindication signifying that voltages V1 and V2 are in agreement. Thevital relay will remain energized so longas transformer T2 remainsunsaturated so that sufficient a.c. voltage is induced in secondarywinding S2.

Let us now assume that the d.c. inputs across terminals l, 2 and 3, 4fall out of agreement due to a change in either voltage V1 or V2. Such acondition may occur due to the fact that the actual speed of the vehicleis either higher than the last received speed command signal or lowerthan the last received speed command signal, or vice versa.

If, for example, the value of voltage V1 becomes somewhat greater thanthat of voltage V2 and a sufficient amount of d.c. current flows throughthe primary winding P2, the core C2 of transformer T2 will becomesaturated. During saturation, the impedance, namely, the inductivereactance of the transformer T2 falls to a minimum value, so that theprimary winding P2 is effectively a short circuit. Under this condition,substantially all the a.c. voltage is dropped across and dissipated inresistor R and little, if any, a.c. voltage is developed across primarywinding P2. Thus, no a.c. voltage is induced into secondary winding S2.The lack of a.c. output voltage causes the vital relay to becomedeenergized and its armature will release, thereby causing the frontcontacts to open. Thus, the opening of the relay contacts causesinterruption of the agreement indication signal when voltages V1 and V2are out of agreement.

Similar action occurs when voltage V2 becomes greater than voltage V1,except that the d.c. current flows in the opposite direction. That is,the d.c. current flows from terminal 3 through the series circuitcausing core C2 to saturate in the opposite direction. Hence, when thedifferential voltage between voltages V1 and V2 causes sufficientcurrent to saturate transformer T2, no a.c. output will be producedthereby signifying a lack of agreement.

It will be appreciated that the amount of d.c. current required to causesaturation is dependent upon the circuit parameters as well as upon themagnetic and electrical characteristics of the transformer T2. It willbe understood that the voltage levels of voltages V1 and V2 will vary inaccordance with the speed of the vehicle and the speed command signals,and, in fact, the voltages will have a zero value when the vehicle isstopped and a stop command signal is being received aboard the vehicle.Thus, the comparator circuit compares the difference in the voltagelevels of voltages V1 and V2 irrespective of their specific amplitudes.In addition, as previously mentioned, no critical circuit or componentfailure is capable of producing or simulating an a.c. output whenvoltages V1 and V2 are not in agreement.

For example, the opening of any lead, terminal or element in the seriescircuit causes the a.c. signal path to be interrupted so that no a.c.signals will be induced into secondary winding S2.

The opening of the primary winding P1 or secondary winding S2 destroysthe transformer action of transformers T1 and T2, respectively.

The shorting of turns between any of the transformer windings reducesthe amplitude of the a.c. output signal and, therefore, such a failureis a safe failure.

If either voltage V1 or V2 becomes short-circuited, a differentialvoltage condition will be produced thereby causing saturation oftransformer T2 so that no a.c. output will be available across secondarywinding S2.

The resistor R is preferably constructed of a carbon composition so thatthis element is incapable of becoming short circuited.

Accordingly, it can be seen that this unique comparator circuit producesan output across the secondary winding S2 when and only when thepotential levels of voltages V1 and V2 are in agreement and no criticalcircuit or component failure is present.

It is readily understood that the magnitudes and polarities of voltagesV1 and V2 are of little or no consequence in that the circuit operateson a differential principle irrespective of magnitude and polarity. Itwill be appreciated that various alterations may be made by personsskilled in the art without departing from the spirit and scope of thisinvention. Therefore it is understood that all changes, equivalents, andmodifications falling within the spirit and scope of the presentinvention are herein meant to be included in the appended claims.

Having thus described my invention, what I claim is:

l. A fail-safe voltage comparator comprising, a pair of input signals tobe compared, a first transformer having a primary and a secondarywinding, a source of a.c. signals coupled to said primary winding andinducing a.c. signals into said secondary winding, one end of saidprimary winding of said second transformer connected in series with oneend of said secondary winding of said 1, wherein each of said pair ofinput signals is a do.

voltage source.

3. A fail-safe voltage comparator as defined in claim 1, wherein saidsecond transformer is a saturable type of transformer.

4. A fail-safe voltage comparator as defined in claim 1, acurrent-limiting resistor is connected in series with said primarywinding of said second transformer and said primary winding of saidfirst transformer.

5. A fail-safe voltage comparator as defined in claim 1, wherein saidsecond transformer includes a saturable core.

6. A fail-safe voltage comparator as defined in claim 5, wherein saidsaturable core of said second transformer is not saturated when saidpair of input signals are in agreement and is saturated when said pairof mg?" UNITED STATES PATENT OFFICE 7 CERTIFICATE OF. CORRECTION PatentNo. 3702939 mud Nov. 14, 1972 Inv.ng r(.) John O. G. Darrow I It iscertified that: error appears in the above-identified patent: and thataid Letters Patent are hereby corrected as shown below:

Column 5, Claim l, Line 5 after the comma insert "a second transformerhaving a primary and a secondary windingfl? Signed and sealed this zmhda of April'l973.

(SEAL) Attest:

EDE'JARD M. FLETCHER, JR. ROBERT GOTTSCHALK Attesting OfficerCommissioner-of Patents (5,69) UNITED STATES PATENT omen I CERTIFICATE OCORRECTION Patent No. 3702939 Dated Nov; 14, 1972 humor) John O. G.Darrow It is certified that error appears in the above-identified patentand that said Letters Patent are hereby corrected an shown below:

Column 5, Claim 1, Line 5 after the comma insert "a second transformerhaving a primary and a secondary winding,"

Signed and sealed this 2mm day of April 1973.

(SEAL) Attest: v EDEJARD M. FLETCHER, JR. ROBERT GOTTSCHALK AttestingOfficer Commissioner-of Patents

1. A fail-safe voltage comparator comprising, a pair of input signals tobe compared, a first transformer having a primary and a secondarywinding, a source of a.c. signals coupled to said primary winding andinducing a.c. signals into said secondary winding, one end of saidprimary winding of said second transformer connected in series with oneend of said secondary winding of said first transformer, the other endof said primary winding of said second transformer connected to one ofsaid pair of input signals, and the other end of said secondary windingof said first transformer connected to the other of said pair of inputsignals so that a.c. output signals are induced in said secondarywinding of said second transformer only when said pair of input signalsare in agreeement and no critical circuit or component failure ispresent.
 2. A fail-safe voltage comparator as defined in claim 1,wherein each of said pair of input signals is a d.c. voltage source. 3.A fail-safe voltage comparator as defined in claim 1, wherein saidsecond transformer is a saturable type of transformer.
 4. A fail-safevoltage comparator as defined in claim 1, a current-limiting resistor isconnected in series with said primary winding of said second transformerand said primary winding of said first transformer.
 5. A fail-safevoltage comparator as defined in claim 1, wherein said secondtransformer includes a saturable core.
 6. A fail-safe voltage comparatoras defined in claim 5, wherein said saturable core of said secondtransformer is not saturated when said pair of input signals are inagreement and is saturated when said pair of input signals are not inagreement.